A role for calreticulin in the adult heart?

The essential role of calcium in cardiac development, function, and disease is incontrovertible. The multiple pathways that regulate Ca2+ homeostasis in the heart are complex and frequently intersecting, involving a large number of myocyte proteins. One protein that is emerging as having a central role in Ca2+ homeostasis and in cardiac development is the ubiquitous chaperone protein of the endoplasmic reticulum (ER), calreticulin. Over 40 different functions have been ascribed to this Ca2+-binding protein, including regulation of gene expression, protein folding, cell adhesion, and autoimmunity (reviewed in ref. 1). In the mouse heart, calreticulin expression is activated at 9.5 days postcoitum (dpc) and remains high until 14.5 dpc. Expression is relatively low by 18 dpc and is barely detectable postnatally. Mice in which the Calreticulin gene has been inactivated die by 14.5 dpc, primarily due to cardiac defects with a marked decrease in ventricular wall thickness (2). Therefore, the importance of calreticulin in cardiac development has been solidified, although its specific role remains unclear. In this issue of JCI, Nakamura and colleagues (3) have forced expression of calreticulin in adult cardiac myocytes in the mouse, to study the sequelae of sustained high expression of this protein after development. These animals exhibit decreased systolic function, chamber dilation, and wall thinning, and they die of sudden cardiac death (3). This premature death is associated with complete heart block. While the mechanisms behind these phenotypes remain somewhat unclear, there is a significant decrease in inward Ca2+ current as well as a decrease in expression of connexins 40 and 43. Consistent with a role for the connexins in the phenotypes of the mice is the finding that inactivation of connexin43 in adult cardiac myocytes results in severe arrhythmias (4). In addition, mice lacking connexin40 exhibit sinoatrial, intra-atrial, and atrioventricular conduction disturbances (5). The complete heart block in calreticulin-overexpressing mice might therefore be explained, at least in part, by secondary changes in expression of other proteins, including the connexins. Since other proteins that have been implicated in cardiac pathogenesis, including SERCA2, phospholamban, calsequestrin, and β-myosin heavy chain, remain unchanged in this model, the decrease in connexins 40 and 43 appears to be a relatively specific consequence of expressing the transgene. Still, reduced connexin levels seem unlikely to explain all of its effects, since the conditional and complete null mice for connexins 40 and 43, respectively, do not exhibit dilation and systolic dysfunction. These interesting findings raise issues about the potential role for calreticulin in the pathogenesis of human arrhythmias. Several recent reviews (1, 6) have sifted through the multitude of roles ascribed to calreticulin, seeking to clarify the relevant functions of this protein. Calreticulin binds Ca2+ through two functionally distinct sites and is localized to the ER, where it is known to function as a lectinlike chaperone for a large number of proteins. One might speculate that calreticulin is involved in the proper folding of these proteins, as has been demonstrated for other membrane-associated proteins (6). However, the mechanism by which calreticulin ensures proper protein folding and the role that Ca2+ plays in this process remain unknown. Given that the ER is one of the largest organelles of the cell and that it is arguably the most important source of intracellular Ca2+, calreticulin is well situated to play an important role in cardiac cell biology.